1. Field of the Invention
The present technology relates to nonvolatile memory cells, and in particular to nonvolatile memory cells subject to the program disturb effect.
2. Description of Related Art
The program operation of a nonvolatile memory cell is complicated by the program disturb effect. Programming refers to adding charge to, or removing charge from, selected memory cells of a memory array, unlike the indiscriminate erase operation which resets typically an entire sector of memory cells to the same charge storage state. The invention encompasses both products and methods where programming refers to making the net charge stored in the charge trapping structure more negative or more positive, and products and methods where erasing refers to making the net charge stored in the charge trapping structure more negative or more positive. In the program disturb effect, programming of a selected cell leads to unwanted programming of unselected memory cells adjacent to the selected cell. In particular, the program disturb effect leads to unwanted programming of memory cells that are: 1) located in columns adjacent to the column including the selected cell and 2) connected to the selected row line (the word line providing a gate voltage to the selected cell). The integrity of the memory array is adversely affected by these problems.
A prior approach of addressing the read disturb effect alleviated the conditions giving rise to the unwanted programming of unselected memory cells. Unselected memory cells are programmed because of an unwanted voltage difference across the bit lines connected to the unselected memory cells which are in the column adjacent to the column including the selected cell. For example, if a bit line voltage is raised to program a memory cell positioned on one side of the bit line, the program disturb effect tends to program the adjacent memory cell on the other side of the bit line as well. The unwanted programming of unselected memory cells is prevented by decreasing the magnitude of the unwanted voltage difference across the bit lines connected to the unselected memory cells which are in the column adjacent to the column including the selected cell. For example, of the two bit lines that are used for accessing the column adjacent to the column including the memory cell selected to be programmed, when a program voltage is applied to one of those two bit lines to program the selected memory cell, the voltage of the other bit line is changed to decrease the unwanted voltage difference.
This prevention mechanism masks the underlying tendency towards the program disturb effect, but does not prevent the underlying tendency leading to the program disturb effect. Because the program disturb effect is an intrinsic effect of many programming mechanisms, it would be advantageous to somehow take advantage of the program disturb effect, rather than simply applying voltages to other bit lines for the sole purpose of counteracting the voltage conditions that give rise to the program disturb effect.